Total Synthesis of Plusbacin A3 and Its Dideoxy Derivative Using a Solvent-Dependent Diastereodivergent Joullié-Ugi Three-Component Reaction

Article abstract of DOI:10.1021/acs.joc.8b00038

Full details of our synthetic studies toward plusbacin A₃ (1), which is a depsipeptide with antibacterial activity, and its dideoxy derivative are described. To establish an efficient synthetic route of 1, a solvent-dependent diastereodivergent Joullié-Ugi three-component reaction (JU-3CR) was used to construct trans-Pro(3-OH) in a small number of steps. Two strategies were investigated toward the total synthesis. In the first synthetic strategy, the key steps were the trans-selective JU-3CR and a macrolactonization at the final stage of the synthesis. The JU-3CR using alkyl isocyanides in 1,1,1,3,3,3-hexafluoroisopropanol provided the trans products, and the coupling of the fragments to prepare the macrocyclization precursor proceeded smoothly. However, attempts toward the macrolactonization did not provide the desired product. Then, the second strategy that included esterification in an initial stage was investigated. Methods for constructing trans-Pro(3-OH) were examined using a convertible isocyanide, which could be converted to a carboxylic acid required for the following amidation. Ester bond formation was achieved through an intermolecular coupling using a hydroxyl-Asp derivative and the corresponding alcohol, and the amidation afforded a linear depsipeptide. The macrolactamization of the linear peptide gave the cyclic depsipeptide, and then the global deprotection accomplished the total synthesis of 1 and its dideoxy derivative.

Full text of DOI:10.1021/acs.joc.8b00038

Subscriber access provided by UNIV OF DURHAM
Article
Total synthesis of plusbacin A3 and its dideoxy derivative using a solvent-
dependent diastereodivergent Joullié–Ugi three-component reaction
Akira Katsuyama, Fumika Yakushiji, and Satoshi Ichikawa
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.8b00038 • Publication Date (Web): 19 Feb 2018
Downloaded from http://pubs.acs.org on February 19, 2018
Just Accepted
“Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted
online prior to technical editing, formatting for publication and author proofing. The American Chemical
Society provides “Just Accepted” as a service to the research community to expedite the dissemination
of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in
full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully
peer reviewed, but should not be considered the official version of record. They are citable by the
Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore,
the “Just Accepted” Web site may not include all articles that will be published in the journal. After
a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web
site and published as an ASAP article. Note that technical editing may introduce minor changes
to the manuscript text and/or graphics which could affect content, and all legal disclaimers and
ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or
consequences arising from the use of information contained in these “Just Accepted” manuscripts.
The Journal of Organic Chemistry is published by the American Chemical Society.
1155 Sixteenth Street N.W., Washington, DC 20036
Published by American Chemical Society. Copyright © American Chemical Society.
However, no copyright claim is made to original U.S. Government works, or works
produced by employees of any Commonwealth realm Crown government in the course
of their duties.

Page 1 of 46
The Journal of Organic Chemistry
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
Total synthesis of plusbacin A3 and its dideoxy
derivative using a solvent-dependent diastereodivergent
Joullié–Ugi three-component reaction
Akira Katsuyama†, Fumika Yakushiji†,‡, and Satoshi Ichikawa*,†, ‡
AUTHOR ADDRESS
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
†Faculty of Pharmaceutical Sciences, Hokkaido University, Kita-12, Nishi-6, Kita-ku, Sapporo 060-
0812, Japan.
‡Center for Research and Education on Drug Discovery, Hokkaido University, Kita-12, Nishi-6, Kita-
ku, Sapporo 060-0812, Japan.
AUTHOR EMAIL ADDRESS
ichikawa@pharm.hokudai.ac.jp
RECEIVED DATE
CORRESPONDING AUTHOR FOOTNOTE
†Faculty of Pharmaceutical Sciences, Hokkaido University, Kita-12, Nishi-6, Kita-ku, Sapporo 060-
0812, Japan.
‡Center for Research and Education on Drug Discovery, Hokkaido University, Kita-12, Nishi-6, Kita-
ku, Sapporo 060-0812, Japan.
ACS Paragon Plus Environment
1

The Journal of Organic Chemistry
Page 2 of 46
ABSTRACT: Full details of our synthetic studies toward plusbacin A3 (1), which is a depsipeptide with
antibacterial activity, and its dideoxy derivative are described. To establish an efficient synthetic route
of 1, a solvent-dependent diastereodivergent Joullié−Ugi three-component reaction (JU-3CR) was used
1
2
3
4
5
6
7
8
9
to construct trans-Pro(3-OH) in a small number of steps. Two strategies were investigated toward the
total synthesis. In the first synthetic strategy, the key steps were the trans-selective JU-3CR and a
macrolactonization at the final stage of the synthesis. The JU-3CR using alkyl isocyanides in
1,1,1,3,3,3-hexafluoroisopropanol provided the trans products, and the coupling of the fragments to
prepare the macrocyclization precursor proceeded smoothly. However, attempts toward the
macrolactonization did not provide the desired product. Then, the second strategy that included
esterification in an initial stage was investigated. Methods for constructing trans-Pro(3-OH) were
examined using a convertible isocyanide, which could be converted to a carboxylic acid required for the
following amidation. Ester bond formation was achieved through an intermolecular coupling using a
hydroxyl-Asp derivative and the corresponding alcohol, and the amidation afforded a linear
depsipeptide. The macrolactamization of the linear peptide gave the cyclic depsipeptide, then the global
deprotection accomplished the total synthesis of 1 and its dideoxy derivative.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
INTRODUCTION
Bacteria have cell walls that protect them from harmful environments, such as increases in turgor
pressure or drug penetration. In particular, peptidoglycan is a main component of the bacterial cell wall,
and its disruption through the inhibition of its biosynthesis is an established strategy for developing
antimicrobial agents. β-Lactams inhibit the transpeptidase that polymerize lipid II, a precursor of
peptidoglycan,1 and vancomycin inhibits lipid II polymerization by binding to the
D-Ala-D-Ala moiety
of lipid II.2 Both classes of antibacterial drugs are frequently used in clinics; however, multiple drug-
resistant bacteria that are resistant to these drugs have been reported.3 Therefore, developing drug
candidates that can control outbreaks of resistant pathogens via different mechanisms of actions is very
important. It is also important to develop antibacterial agents that are less susceptible to these drug
ACS Paragon Plus Environment
2

Page 3 of 46
The Journal of Organic Chemistry
resistances. Some natural products that bind to non-protein peptidoglycan precursors including lipid II,
such as vancomycin, ramoplanin and teixobactin, are currently known to be less susceptible to the
generation of drug resistance.4 Hence, this series of compounds that inhibit peptidoglycan biosynthesis
via binding to its precursors can be promising antibacterial leads.
1
2
3
4
5
6
7
8
9
Plusbacin A3 (1, Figure 1) was isolated from Pseudomonas sp. PB-6250 in 1992 as a potential
antimicrobial agent against methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin
resistant Enterococci (VRE).5 It has been reported that 1 inhibits lipid II polymerization, although the
details of the mechanism of action remain to be elucidated.6 Hence, the development of efficient
synthetic route and an investigation of the mechanism of action of 1 are significant challenges in
developing new antibacterial candidates.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
H2N
HN
NH
Arg
D
-Ser
Pro(3-OH)
HO
O
O
OH
H
N
N
HO
N
H
N
H
O
O
O
N
HO2C
Asp(β-OH) H
OH
O
NH
O
Pro(3-OH)
HO
O
N
N
H
D-Ala
HO2C
Asp(β-OH)
O
O
HO
allo-
D-Thr
ester
3-hydroxyisohexadecanoic acid
plusbacin A3 (1)
Figure 1. Chemical structure of plusbacin A3 (1).
Plusbacin A3 is a cyclic depsipeptide that contains two trans-3-hydroxyl L-prolines [Pro(3-OH)], L-
and
cyclic peptide core skeleton. To achieve an efficient total synthesis of 1, the construction of Pro(3-OH)
is a key step. Park and co-workers reported that -gluconolactone could be converted to Pro(3-OH) via
D
-β-hydroxy-aspartic acids [Asp(β-OH)], and 3-hydroxyisohexadecanoic acid in its 28-membered
D
an allylglycine derivative in 14 steps.7 The conversion from but-3-yn-1-ol to Pro(3-OH) via a Sharpless
asymmetric epoxidation in 15 steps was described by Chandrasekhar’s group.8 These methodologies are
ACS Paragon Plus Environment
3

The Journal of Organic Chemistry
Page 4 of 46
well-designed; however, diastereoselective transformation in short steps is required for the synthesis of
1. Since the Pro(3-OH) is also included in some peptide-based natural products,9 it is important to
develop an efficient synthetic methodology for constructing the Pro(3-OH). As a result, a Joullié−Ugi
1
2
3
4
5
6
7
8
9
three-component reaction (JU-3CR),10 which enables the construction of Pro(3-OH) in one step, was
selected for efficiency. The JU-3CR was first reported in 1982 as a three-component coupling reaction
using the combination of an isocyanide, a cyclic imine and a carboxylic acid (Scheme 1(a)).10a This
reaction has the advantage that it can synthesize Pro(3-OH) with simultaneous amide bond formation at
both the C- and N-termini. In our previous work, a diastereodivergent JU-3CR that can regulate the
trans/cis selectivity through the selection of the proper solvent was developed.11 Non-polar solvents,
such as toluene, provided cis products, while polar 1,1,1,3,3,3-hexafluoroisopropanol (HFIP) afforded
predominantly trans products with the same combination of substrates (Scheme 1(b)). This newly
developed solvent-dependent diastereodivergent JU-3CR was thought to be effective for the rapid
construction of the two Pro(3-OH) moieties of 1.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Scheme 1. Joullié−Ugi three-component reaction.
(a) Joullié et al.
trans
cis
O
O
O
O
OPh
OPh
OPh
N C
N
H
Ph
N
H
Ph
MeOH
58%
+
N
O
N
N
trans/cis = 45/55
Ph
OH
(b) Our previous work
OTIPS
O
O
OPh
OPh
N C
toluene
HFIP
60%
trans/cis
= 82/18
N
H
N
H
N
N
N
Ph
63%
trans/cis
= 15/85
O
Ph
O
O
Ph
OH
To date, total syntheses of plusbacin A3 (1) have been reported by VanNieuwenhze and co-workers in
2007,12 and by our group in 2017.13 We describe herein the full details of our effort to accomplish the
ACS Paragon Plus Environment
4

Page 5 of 46
The Journal of Organic Chemistry
total synthesis of 1 via the solvent-dependent diastereodivergent JU-3CR.11 During this study, a dideoxy
1
2
3
derivative was also synthesized.
4
5
6
7
8
RESULTS AND DISCUSSION
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Upon utilizing the JU-3CR for the total synthesis of 1, the cyclic imine 4 was prepared as shown in
Scheme 2. (S)-4-Amino-2-hydroxybutanoic acid (2) was treated with hexamethyldisilazane (HMDS)
and a catalytic amount of TMSCl to obtain a lactam,14 and its secondary alcohol was protected as a
triisopropylsilyl ether to give 3 in 76% yield over two steps. Lactam 3 was cleanly converted to cyclic
imine 4 (84% yield) using the hydrozirconium reagent reported by Schwartz.15 Imine 4 existed in
equilibrium with trimer 5 in several organic solvents.
Scheme 2. Synthesis of cyclic imine 4.
Isocyanide 9, which is composed of the Asp(β-OH) residue at the C-terminal of one of the Pro(3-OH)
moieties, was prepared as shown in Scheme 3. Known alcohol 616 was protected with a triisopropyl silyl
group, and ozonolysis of the olefin followed by oxidation gave carboxylic acid 7 in 84% yield over
three steps. As a β-hydroxy carboxylic acid protecting group, a cyclohexyl ester was selected to avoid
an intramolecular cyclization. The condensation reaction between 7 and cyclohexanol using 1-(3-
dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (EDCI) and DMAP and a Mitsunobu reaction
ACS Paragon Plus Environment
5

The Journal of Organic Chemistry
Page 6 of 46
using DEAD and PPh3 were initially examined; however, only a moderate amount of 8 was obtained.
Therefore, a stepwise construction of 8 was investigated by using 3-bromocyclohex-1-ene as the
coupling partner. Alkylation of 7 with 3-bromocyclohex-1-ene under basic conditions and
hydrogenation of the resulting cyclohexenyl ester using H2 and the Pd/Fibroin complex (Pd/Fib)17
provided cyclohexyl ester 8 in excellent yield (83% and 98% yield for the two steps, respectively). The
Boc protecting group of 8 was removed under acidic conditions, and the liberated amine was formylated
using N-formylsaccharin.18 The remaining primary alcohol was oxidized to the carboxylic acid, which
was allylated (24% yield over four steps), and the corresponding allyl ester was then treated with
triphosgene to provide isocyanide 9. In addition, 2,2,2-trichloroethoxycarbonyl (Troc)-protected
isocyanide 10 was synthesized in four steps from 8.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Scheme 3. Synthesis of isocyanides 9 and 10.
ACS Paragon Plus Environment
6

Page 7 of 46
The Journal of Organic Chemistry
In our initial exploration of the JU-3CR for the total synthesis of 1, the scope and limitations of the
1
2
isocyanides were investigated (Scheme 4). Considering a direct conversion, the combination of cyclic
imine 4, isocyanide 9 and protected Ser 11 was examined in HFIP. However, this reaction provided a
complex mixture (ex. 1). Because steric hindrance was thought to be the reason why the reaction did not
proceed, simple isocyanide 12 containing a β carbonyl moiety was used instead of 9, but this
combination also provided a complex mixture of products (ex. 2). Zhu and coworkers reported that α-
isocyanocarboxamide can be converted to an oxazoline by an intramolecular cyclization.19 In the
analysis of the reaction mixture with mass spectrometry, a peak at 624.46 [(M+Na)+] corresponding to
complex 14, which is composed of oxazoline and 4, was detected. Oxazoline was incorporated into the
equilibrium of the trimer formation, and generated complex 14 could interfere in the progression of the
reaction (Scheme 5). In the case of Troc-protected alkyl isocyanide 10, which lacks the carbonyl group
and thereby cannot form a trimer such as 14, desired trans-13 was obtained in 57% yield as the major
product over cis-13 (17%) (ex. 3). These investigations suggested that the carbonyl on the β position of
the isocyano group should be avoided in the JU-3CR.
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Scheme 4. Initial scope and limitations of isocyanides for the JU-3CR.
O
O
TIPSO
HO2C
HFIP
(ex. 1)
complex mixture
4 +
CO2Cy
+
CN
NHBoc
OTIPS
OtBu
9
11
11
O
HFIP
(ex. 2)
complex mixture
4 +
+
CN
12
TIPSO
N
TIPSO
N
OTroc
CO2Cy
TIPSO
TIPSO
NHBoc
NHBoc
HFIP
O
NH
O
NH
(ex. 3)
4 +
+
11
+
CN
O
O
trans/cis
= 77/23
TIPSO
OTroc
TIPSO
OTroc
OTIPS
10
CO2Cy
trans-13 (57%)
CO2Cy
cis-13 (17%)
Scheme 5. Generation of trimer 14 from isocyanide 12.
ACS Paragon Plus Environment
7

The Journal of Organic Chemistry
Page 8 of 46
O
O
C
OtBu
1
2
3
4
5
6
TIPSO
OtBu
N
N
N
N
OtBu
12
O
N
OTIPS
N
N
TIPSO
OTIPS
N
N
OTIPS
7
8
9
14
MS 624.46 [(M+Na)+]
4
TIPSO
5
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
With these initial investigations into the JU-3CR in hand, a retrosynthetic analysis of plusbacin A3
(1) was conducted as shown in Scheme 6. The 28-membered macrocycle was to be synthesized via a
macrolactonization of carboxylic acid 15, which was disconnected to peptide segments 16, 17 and 18
via amidation. Segment 16 was prepared via the JU-3CR using alkyl isocyanide 10, cyclic imine 4 and
carboxylic acid 11, and 17 can be constructed by the same strategy via the JU-3CR using alkyl
isocyanide 19, cyclic imine 4 and carboxylic acid 20 followed by amidation with 21. Amide 18,
containing a lipid side chain, can be generated by the coupling of 22 and 23.
Scheme 6. Retrosynthetic analysis of plusbacin A3 (1).
ACS Paragon Plus Environment
8

Page 9 of 46
The Journal of Organic Chemistry
HN
HN
NH2
HN
HN
NH2
amidation
1
2
3
4
5
6
7
8
HO
HO
O
O
O
O
OH
OH
H
N
H
N
N
N
HO
HO
N
H
N
H
O
N
H
N
H
O
O
NH
O
O
OH
H
N
O
OH
H
N
N
N
O
HO2C
O
NH
HO2C
O
O
HO
HO
HO2C
HO
O
N
H
N
H
CO2H
O
HO
O
HO
HO2C
O
amidation
9
plusbacin A3 (1)
macrolactonization
15
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
N3
O
TIPSO
building blocks for 16
amidation
NC
N
TIPSO
JU-3CR
TIPSO
HO2C
NH2
O
OTIPS
TIPSO
CyO2C
OTroc
H
TIPSO
O
NH
N
N
O
O
N
H
O
NHBoc
O
10
4
11
O
OTIPS
N
TIPSO
CyO2C
O
CyO2C
JU-3CR
building blocks for 17
O
BocHN
CbzHN
CO2H
16
NH2
TIPSO
O
17
N
TrocO
NC
BocHN
20
amidation
H
N
CyO2C
OTIPS
HO
CO2H
19
4
21
O
TIPSO
building blocks for 18
O
H2N
CO2Bn
18
olefin cross metathesis
O
TIPSO
22 8
23
First, the building blocks required for the synthesis of 1 (11, 19, 20, 21, 22, and 23) were prepared.
Suitably protected serine 11 was synthesized in three steps from 24 by a Boc protection of the amino
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
group, triisopropylsilylation and hydrolysis of the silyl ester (Scheme 7).
Scheme 7. Synthesis of carboxylic acid 11.
1) (Boc)2O, aq. NaHCO3, THF
2) TIPSCl, imidazole, CH2Cl2
3) K2CO3, MeOH, THF, H2O
HO
TIPSO
HO2C
quant. over 3 steps
HO2C
NH2
NHBoc
24
11
The Boc protecting group on known alcohol 2520 was removed under acidic conditions and the
liberated amine was formylated. The primary alcohol was Troc-protected to give 26 in 61% yield over
three steps. Treatment of 26 with triphosgene provided isocyanide 19 in 84% yield (Scheme 8).
ACS Paragon Plus Environment
9

The Journal of Organic Chemistry
Page 10 of 46
1
2
3
Scheme 8. Synthesis of isocyanide 19.
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Protected D-Asp(β-OH) 21 was synthesized from ent-8. The isopropylidene group of ent-8 was
removed, and the generated primary alcohol was oxidized to carboxylic acid 27. The carboxylic acid of
27 was alkylated using allyl bromide, and the Boc group of 28 was removed to afford amine 21
(Scheme 9).
Scheme 9. Synthesis of amine 21.
CrO3, H2SO4,
H2O, acetone, 0 °C
allyl bromide
Cs2CO3
O
CO2H
CO2Cy
CO2Cy
OTIPS
ent-8
BocHN
N
Boc
DMF
92%
79% brsm
(45% conversion)
OTIPS
27
O
O
O
NH2
OTIPS
25% TFA/CH2Cl2
quant.
O
CO2Cy
OTIPS
BocHN
CyO2C
21
28
Suitably protected allo-D-Thr 23 was prepared by a sequential Boc, benzyl and triisopropylsilyl
protections of 29 to give 30, which was followed by Boc deprotection of 30 under acidic conditions
(Scheme 10).
ACS Paragon Plus Environment
10

Page 11 of 46
The Journal of Organic Chemistry
1
2
3
Scheme 10. Synthesis of protected allo-D-Thr 23.
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Synthesis of amide 18, containing the lipid side chain, began from β-lactone 22, which had been
constructed via an enantioselective [2+2]cycloaddition reaction using aldehyde 31 and a ketene
generated from acetyl bromide in the presence of the optically active Al(Ⅲ)–triamine complex21 (94%
yield, 92% ee) (Scheme 11). Obtained 22 and 2-methylbut-3-en-2-ol were treated with Grubbs 2nd
generation catalyst22 to give olefin 32 in 81% yield. When 3-methylbut-1-en was used for this cross-
metathesis reaction, the double bond was isomerized, and a complex mixture of products containing
shorter carbon chains was produced. Metathesis product 32 was dehydrated using Burgess reagent23 to
give 33. The diene of 33 was then hydrogenated under PtO2 catalysis to provide desired 34 in 66% yield
over two steps. The use of Pd/C as a catalyst led to a complex mixture due to reduction of the C-O bond.
Lactone 34 was converted to carboxylic acid 35 in 80% yield, and it was then condensed with 23 using
EDCI, 1-hydroxy-7-azabenzotriazole (HOAt) and NaHCO3 to give amide 36 in 86% yield. The benzyl
group of 36 was removed by hydrogenolysis catalyzed by Pd/C to afford 18 in 99% yield.
Scheme 11. Synthesis of amide 18.
ACS Paragon Plus Environment
11

The Journal of Organic Chemistry
Page 12 of 46
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
The synthesized building blocks 4, 10 and 11 were used for the key JU-3CR (Scheme 12). In our
previous report,11 the solvent-dependent diastereodivergent JU-3CR was examined in either toluene or
HFIP. The non-polar solvent (toluene) predominantly provided cis products, while the polar solvent
(HFIP) produced trans products. Thus, HFIP was selected as a solvent of the preparation of trans
tripeptide 13. The reaction of 4, 10 and 11 in HFIP at room temperature for 19.5 h smoothly provided
trans-13 as well as cis-13 in yields of 57% and 17%, respectively, and as expected, the trans product
was obtained as a major product. These two diastereomers were easily separated by silica gel column
chromatography. Removal of the Troc protecting group of trans-13 was investigated. As shown in
Scheme 12, entry 1, the conditions using a zinc-copper couple in the presence of ammonium chloride
provided only dechlorinated derivative 38. The reaction presumably proceeded via protonation of the
organo zinc intermediate. Therefore, we examined the reaction in the absence of a proton source under
single-electron reduction conditions using SmI2. Although desired alcohol 37 was obtained in 50% yield
(entry 2), 38 was still produced under these conditions. Hence, the solvolysis of the Troc group was
conducted using Sm and I2 in MeOH (entry 3).24 The methanolysis cleanly afforded desired 37 in 95%
yield, the primary alcohol of 37 was oxidized to a carboxylic acid, and subsequent alkylation afforded
ACS Paragon Plus Environment
12

Page 13 of 46
The Journal of Organic Chemistry
allyl ester 39. The Boc protecting group of 39 was removed by TFA to provide tripeptide 16 in
1
2
3
quantitative yield.
4
5
6
7
8
Scheme 12. Synthesis of tripeptide 16 via a JU-3CR.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
OTIPS
TIPSO
N
TIPSO
N
TIPSO
HO2C
TIPSO
TIPSO
NHBoc
NHBoc
N
NHBoc
HFIP
O
NH
O
NH
+
11
4
O
O
NC
TIPSO
OTroc
TIPSO
CyO2C
OTroc
TIPSO
OTroc
CyO2C
trans-13 (57%)
CyO2C
cis-13 (17%)
10
conditions
conditions
TIPSO
N
TIPSO
N
yield (%)
37
38
79
50
–
reagents
entry
TIPSO
TIPSO
NHBoc
NHBoc
–
O
NH
O
1
2
3
Zn/Cu, NH4Cl, MeOH
SmI2, THF
+
O
O
NH
50
95
TIPSO
CyO2C
OH
TIPSO
CyO2C
O
O
CHCl2
Sm, I2, MeOH
O
37
38
1) DMP, THF
2) NaClO2, NaH2PO4
2-methylbut-2-ene
tBuOH/THF/H2O
3) allyl bromide, Cs2CO3
DMF
53% over 3 steps
TIPSO
N
TIPSO
NHBoc
O
NH
25% TFA/CH2Cl2
quant.
O
16
TIPSO
O
CyO2C
O
39
The JU-3CR in HFIP was examined for preparing the trans product using building blocks 19, 4 and
20 (Scheme 13). As a result, desired trans-40 was obtained in moderate yield (43%) along with cis-40
(36%), although the reason for the decreased selectivity was unclear. After the separation of the
products of the JU-3CR by silica gel column chromatography, the Troc and Cbz groups of trans-40
ACS Paragon Plus Environment
13

The Journal of Organic Chemistry
Page 14 of 46
were removed by solvolysis mediated by Sm and I2 in MeOH and hydrogenolysis, respectively, and the
liberated amine was converted to an azide25 to afford 41 in 88% yield over three steps. The primary
alcohol of 41 was oxidized by Dess-Martin periodinane (DMP), which was then treated with sodium
chlorite to afford carboxylic acid 42, which was condensed with amine 21 to provide tetrapeptide 17 in
44% yield over three steps. The temporal transformation to the azide as 41 was nesessary, because the
oxidation to an aldehyde from the primary alcohol provided a hemiaminal intermediate, and the second
oxidation generated a γ-lactam (Scheme 14).
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Scheme 13. Synthesis of tetrapeptide 17 via the JU-3CR.
CbzHN
CbzHN
TrocO
O
O
OTIPS
OTIPS
TrocO
HFIP
N
H
O
N
H
O
19 + 4 + 20
+
N
N
BocHN
BocHN
trans-40 (43%)
cis-40 (36%)
1) Sm, I2, MeOH
2) H2, Pd black
AcOH/MeOH
3)
O
NH
HSO4
N3
S
N
-
O
CuSO4, K2CO3, MeOH
88% over 3 steps
N3
N3
21
EDCI, HOAt
NaHCO3
O
O
OTIPS
OTIPS
HO
1) DMP, THF
N
H
O
HO2C
N
H
O
17
N
N
2) NaClO2, NaH2PO4
2-methylbut-2-ene
tBuOH/THF/H2O
44% over 3 steps
BocHN
BocHN
41
42
Scheme 14. Generation of a γ-lactam from a Cbz-protected amino alcohol.
CbzHN
HO
CbzHN
O
O
O
O
OTIPS
OTIPS
OTIPS
OTIPS
CbzN
CbzN
O
DMP
THF
[O]
N
H
O
N
H
O
N
H
O
N
H
O
OH
N
O
N
N
H
N
BocHN
BocHN
BocHN
BocHN
With the three segments in hand, their coupling to form linear peptide 46 and its subsequent
cyclization were investigated (Scheme 15). The Boc group of tetrapeptide 17 was removed to give
ACS Paragon Plus Environment
14

Page 15 of 46
The Journal of Organic Chemistry
amine 43, which was condensed with 18 to provide hexapeptide 44 in 63% yield over two steps. The
allyl group of 44 was removed with Pd(PPh3)4 and morpholine, and the liberated amine was converted to
a Boc-protected guanidine using Goodman’s reagent26 to afford 45 in 73% yield over three steps from
44. The amide bond formation between 45 and 16 was promoted by EDCI, HOAt and NaHCO3 and was
followed by the removal of the allyl group to provide carboxylic acid 46. Finally, the hydroxy
carboxylic acid moiety was converted to an acyl fluoride with fluoro-N,N,N′,N′-
bis(tetramethylene)formamidinium hexafluorophosphate (TFFH),27 and the product was subjected to
macrolactonization. However, the desired depsipeptide was not obtained except unanalyzable
compounds. Although other macrolactonization conditions were extensively explored, only hydroxy
carboxylic acid 46 was recovered.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Scheme 15. Attempts toward the total synthesis of plusbacin A3 (1).
N3
O
O
OTIPS
H
N
O
N
H
O
BocHN
HN
NBoc
O
OTIPS
N
N3
O
CyO2C
O
1) Pd(PPh3)4
OTIPS
H
morpholine, THF
2) H2, Pd black
AcOH/MeOH
N
O
OTIPS
RHN
O
N
H
H
N
EDCI, HOAt
NaHCO3, CH2Cl2
HO2C
O
OTIPS
N
17: R = Boc
43: R = H
N
H
O
CyO2C
HO
25%
TFA/CH2Cl2
O
OTIPS
O
N
H
N
3) (BocHN)2C=NTf
Et3N, CH2Cl2
O
63% over 2 steps
CyO2C
O
N
H
H
N
H
N
HO
73% over 3 steps
O
TIPSO
HO
CO2H
N
H
O
TIPSO
O
TIPSO
44
45
18
F
N
N
-
1) 16, EDCI, HOAt
NaHCO3, CH2Cl2
2) Pd(PPh3)4
PF6
BocHN
HN
NBoc
BocHN
HN
NBoc
TFFH
OTIPS
morpholine, THF
TIPSO
N
TIPSO
O
O
O
O
OTIPS
H
N
H
N
TIPSO
N
TIPSO
N
H
N
H
O
N
H
N
H
O
O
NH
TFFH, Et3N
CH2Cl2
O
OTIPS
O
NH CyO2C
O
OTIPS
N
N
CyO2C
O
O
O
O
H
N
H
N
TIPSO
HO
CO2H
TIPSO
CyO2C
O
O
N
H
N
H
CyO2C
O
TIPSO
O
TIPSO
46
Because the macrolactonization did not proceed, an intermolecular esterification using tripeptide 47
and alcohol 48 was examined to elucidate the reactivity of the esterification under a variety of
ACS Paragon Plus Environment
15

The Journal of Organic Chemistry
Page 16 of 46
conditions (Table 1). However, desired product 49 was not obtained, while elimination product 50 was
1
2
3
obtained in moderate yield when EDCI and DMAP were used. These examinations suggested that the
4
5
6
bulky environment prevented ester bond formation. Small substrates, such as L-threo-β-hydroxy-Asp,
7
8
were preferable when this esterification was conducted at an early stage in the synthesis.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Table 1. Examination of intermolecular esterification using tripeptide 47 and alcohol 48.
TIPSO
TIPSO
N
TIPSO
NHBoc
N
TIPSO
O
NH
NHBoc
HO
O
see table
O
O
+
O
NH
O
TIPSO
CyO2C
O
O
8
TIPSO
CyO2C
CO2H
O
O
47
48
49
entry
conditions
EDCI, DMAP, CH2Cl2
T3P, N-methylmorpholine, DMF
result
TIPSO
N
1
2
3
4
5
6
50 (39%)
no reaction
TIPSO
NHBoc
O
NH
O
TFFH, N-methylmorpholine, CH2Cl2 decomp.
O
O
O
DEAD, PPh3, benzene
Tsunoda reagent, benzene
Dy(OTf)3, toluene, reflux
no reaction
no reaction
no reaction
CyO2C
O
50
O
O
P
nPr
P
O
CN
nPr
O
Bu3P
P
O
O
nPr
Tsunoda reagent
T3P
Considering these results, including several failures toward the total synthesis of 1, the synthetic
strategy was revised as shown in Scheme 16. Plusbacin A3 (1) would be generated by
macrolactamization via amide bond formation between D-Ser and D-threo-β-hydroxy-Asp in the same
way as was reported by VanNieuwenhze et al.12 Linear peptide 52 can be incorporated into the structure
by amidation using depsipeptide 54 and pentapeptide 56. Depsipeptide 54 was to be synthesized via a
JU-3CR followed by amidation using an amine containing the appropriate ester moiety. Although the
JU-3CR products from using isocyanide 10 were amides (as shown in Schemes 4 and 12), the
ACS Paragon Plus Environment
16

Page 17 of 46
The Journal of Organic Chemistry
carboxylic acids derived from the JU-3CR products were desirable for the sequential amidation. The
convertible isocyanides were selected to address this point because they allow the transformation of the
JU-3CR products to the carboxylic acids.28 Pentapeptide 56 can be constructed via the same strategy, a
JU-3CR followed by amidation. As described in later, the synthesis of dideoxy derivative 51 was also
planned according to the retrosynthetic analysis of 1.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Scheme 16. Retrosynthetic analysis of 1 and dideoxy derivative 51. The gray circles indicate the
deoxygenated fragments.
HN
HN
NH2
HN
HN
NH2
macrolactamization
HO
O
O
HO
O
OH
H
N
O
O
OH
N
HO
HO
NH2
H
N
N
H
O
N
HO
N
H
N
H
O
O
NH
O
N
R1
O
HO2C
O
N
O
R1
O
NH
HO2C
H
N
O
O
H
N
R1
R1
N
H
O
N
H
O
HO2C
O
HO
O
HO
HO2C
O
amidation
plusbacin A3 (1): R1 = OH
dideoxy derivative 51: R1 = H
52: R1 = OTIPS
53: R1 = H
CbzHN
NCbz
BnO
amidation
HN
O
N
TIPSO
JU-3CR
amidation
O
NHBoc
OTIPS
H
N
O
O
N
O
NH
+
H
N
O
O
R1
R2O2C
R1
O
OH
O
R2O2C
JU-3CR
O
O
H2N
N
H
TIPSO
56: R1 = OTIPS, R2 = Cy
57: R1 = H, R2 = Bn
54: R1 = OTIPS, R2 = Cy
55: R1 = H, R2 = Bn
First, a JU-3CR was examined to construct the carboxylic acid containing the Pro(3-OH) fragment
(Scheme 17). In addition to cyclic imine 4 and carboxylic acid 58, 2-isocyanophenyl acetate (59),28a as
the isocyanide component, was selected for the generation of the protected carboxylic acid via an N-acyl
oxazolidinone intermediate. The reaction was conducted in HFIP to obtain the trans-JU-3CR product,
but desired trans-60 was generated in 22% yield as a minor product. The obtained diastereomers were
ACS Paragon Plus Environment
17

The Journal of Organic Chemistry
Page 18 of 46
separated by silica gel column chromatography, and the acetyl group of trans-60 was removed using
SmCl3. Treatment of the resulting phenol with N,N'-carbonyldiimidazole (CDI) produced N-acyl
oxazolidinone intermediate 61,28b and the condensation with allyl alcohol provided dipeptide 62. This
cis-selective conversion can be explained by the lower nucleophilicity of the aryl isocyanide containing
the sp2 carbon atom adjacent to the isocyano group from our previous study.10 To optimize the trans/cis
selectivity, electron-rich sp3 alkyl isocyanide 6328c was selected for the diastereoselective JU-3CR. In
HFIP, desired trans-JU-3CR product 64 was obtained as a major product in 51% yield, while cis-64 was
isolated in 37% yield. The obtained diastereomers were separable by silica gel column chromatography.
Treatment of trans-64 with t-BuOK and MS (4Å) in THF afforded oxazolidinone intermediate 65,
which was hydrolyzed to carboxylic acid 66.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Scheme 17. JU-3CR for the preparation of Pro(3-OH) derivatives 62 and 66.
BnO
BnO
N
BnO
N
BnO
N
4 +
TIPSO
HN
TIPSO
O
1) SmCl3-6H2O
MeOH
allyl alcohol
DMAP
TIPSO
HO2C
NHBoc
NC
NHBoc
NHBoc
NHBoc
HFIP
O
O
O
O
O
O
58
59
N
O
75%
over 2 steps
2) CDI, CH2Cl2
O
AcO
AcO
+
trans-60 (22%)
+ cis-60 (64%)
61
62
BnO
BnO
N
BnO
N
N
O
TIPSO
TIPSO
O
tBuOK, MS4A
THF, 0 °C
LiOH
HFIP
NHBoc
NHBoc
4 + 58
TIPSO
NHBoc
O
O
O
NH
THF/H2O
N
O
+
PhO2CO
63
O
OH
O
48%
over 2 steps
OCO2Ph
NC
trans-64 (51%)
+ cis-64 (37%)
65
66
Then, the ester bond formation was examined in an earlier stage of this second synthetic pathway
(Scheme 18). The carboxylic acid of 35 was alkylated using allyl bromide and Cs2CO3, and acylation of
the remaining hydroxy group with suitably protected L-Asp(β-OH) (ent-27)12 using EDCI and DMAP
smoothly provided 67 in 58% yield over two steps. The Boc protecting group was removed under acidic
ACS Paragon Plus Environment
18

Page 19 of 46
The Journal of Organic Chemistry
conditions to afford ester 69, which was condensed with carboxylic acid 66 to afford 71 in 86% yield
over two steps, and the removal of the allyl group afforded carboxylic acid 54.
1
2
3
4
5
6
7
8
Scheme 18. Synthesis of depsipeptides 54 and 55.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
To examine the JU-3CR to obtain tripeptide 76, protected dipeptide 74 was first synthesized from
allo-D-Thr (29) via the installation of suitable protecting groups, condensation with H-D-Ala-OBn, and
removal of the benzyl group of 73.
Cyclic imine 4, alkyl isocyanide 63 and carboxylic acid 74 and HFIP were selected for the trans-
selective JU-3CR in a similar manner to the synthesis of 64. As a result, trans-75 was generated in 49%
yield as the major product over cis-75, and trans-75 was separated by silica gel column chromatography.
Pure trans-75 was subjected to basic conditions to generate carboxylic acid 76 in 72% yield over two
steps (Scheme 19).
Scheme 19. Synthesis of tripeptide 76 via a JU-3CR.
ACS Paragon Plus Environment
19

The Journal of Organic Chemistry
Page 20 of 46
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
The synthesis of pentapeptide 56 is shown in Scheme 20. The carboxylic acid of 77 was converted
to the allyl ester, the Boc group was removed, and the product was condensed with a suitably protected
L
-Arg to afford dipeptide 79. The acidic removal of the Boc group produced amine 81, which was
condensed with tripeptide 76 using EDCI, HOAt and i-Pr2NEt in THF to afford pentapeptide 83 in 90%
yield. The Boc group of 83 was removed under acidic conditions to generate amine 56.
Scheme 20. Synthesis of pentapeptides 56 and 57.
ACS Paragon Plus Environment
20

Page 21 of 46
The Journal of Organic Chemistry
1
2
3
4
5
6
7
8
9
1) allyl bromide
Cs2CO3, DMF
2) 25% TFA/CH2Cl2 or
4 M HCl/AcOEt
CbzHN
HN
NCbz
CbzHN
HN
NCbz
O
25% TFA/CH2Cl2
for 81
NHBoc
R1
O
O
HO
R2O2C
H
N
H
N
3) Boc-Arg(Cbz)2-OH
EDCI, HOBt
iPr2EtN, CH2Cl2
4 M HCl/AcOEt
for 82
O
NHBoc
O
NH2
O
O
R2O2C
R1
R2O2C
R1
79: R1 = OTIPS, R2 = Cy, 63% over 3 steps
80: R1 = H, R2 = Bn, 89% over 3 steps
77: R1 = OTIPS, R2 = Cy
78: R1 = H, R2 = Bn
81: R1 = OTIPS, R2 = Cy
82: R1 = H, R2 = Bn, HCl salt
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
CbzHN
HN
NCbz
CbzHN
HN
NCbz
O
O
O
O
OTIPS
H
N
OTIPS
76, EDCI
HOAt, iPr2EtN
H
N
25% TFA/CH2Cl2
O
N
H
O
O
N
H
O
O
O
N
N
THF
R2O2C
R1
R2O2C
R1
O
O
BocHN
H2N
N
H
N
H
TIPSO
83: R1 = OTIPS, R2 = Cy, 90%
84: R1 = H, R2 = Bn, 86%
TIPSO
56: R1 = OTIPS, R2 = Cy
57: R1 = H, R2 = Bn
Synthesized depsipeptide 54 and pentapeptide 56 were condensed using EDCI and HOAt in the
presence of i-Pr2EtN and desired linear peptide 85 was obtained in 64% yield. The allyl ester of 85 was
converted to a carboxylic acid, and the Boc protecting group was removed by using 25% TFA/CH2Cl2.
The macrolactamization using EDCI and HOAt produced cyclic depsipeptide 87 in 60% yield over three
steps. Finally, 87 was globally deprotected using HF and anisole, and the crude product was purified by
reversed-phase HPLC to complete the synthesis of plusbacin A3 (1).
The synthesis of dideoxy derivative 51 was also conducted to investigate the effects of a hydroxy
group on the Asp residues on the biological activities. The same synthetic strategy as was used for 1 was
applied by using suitably protected D- or L-Asp, instead of protected D- or L-threo-β-hydroxy-Asp, and
desired 51 was obtained as expected (Schemes 16–21).
Scheme 21. Completion the total syntheses of 1 and its dideoxy derivative 51.
ACS Paragon Plus Environment
21

The Journal of Organic Chemistry
Page 22 of 46
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
CONCLUSIONS
In conclusion, our synthetic efforts toward the total synthesis of plusbacin A3 (1) and its dideoxy
derivative 51 were described. The total syntheses of 1 and 51 have been accomplished via a solvent-
dependent diastereodivergent Joullié−Ugi three-component reaction (JU-3CR). This reaction, which
used an electron-rich sp3 alkyl isocyanide in HFIP, allowed the construction of the trans-3-hydroxyl
proline in one step. Moreover, alkyl convertible isocyanides enabled the subsequent transformation to
carboxylic acids for the efficient amidation to afford the desired depsipeptide and pentapeptide.
Although macrolactonization in the final stage in the first synthetic strategy failed due to the steric
environment, the ester bond formation between a simple Asp derivative and an alkyl alcohol in an
earlier stage proceeded smoothly in the second approach. Macrolactamization of the linear peptide
followed by global deprotection provided plusbacin A3 (1) and its dideoxy derivative 51. Studies on a
ACS Paragon Plus Environment
22

Page 23 of 46
The Journal of Organic Chemistry
broad range of structure-activity relationships and the mechanism of action of 1 are now in progress and
1
2
3
will be reported in due course.
4
5
6
7
8
9
EXPERIMENTAL SECTION
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
General Information.
All reactions, except those carried out in an aqueous phase, were performed under an argon atmosphere
unless otherwise noted. Isolated yields were calculated by weighing the products. The weights of the
starting materials and the products were not calibrated. Materials were purchased from commercial
suppliers and used without further purification unless otherwise noted. Solvents were distilled according
to the standard protocols. Analytical thin layer chromatography (TLC) analyses were performed on
Merck silica gel 60F254 plates. Normal-phase column chromatography separations were performed on
Merck silica gel 5715 or Kanto Chemical silica gel 60N (neutral). Flash column chromatography
separations were performed on Fuji Sylysia silica gel PSQ 60B. SH-silica gel column chromatography
1
separations were performed on Fuji Sylysia Scavenger SH Silica. H NMR spectra were acquired in
CDCl3, DMSO-d6, CD3CN or D2O with signals referenced to TMS (0.00 ppm) using JEOL ECA 500
(500 MHz), JEOL ECS 400 (400 MHz) or JEOL ECX 400P (400 MHz) spectrophotometers unless
13
otherwise noted. C NMR spectra were acquired in CDCl3, DMSO-d6, CD3CN or D2O with signals
referenced to residual solvent peaks using JEOL ECA 500 (125 MHz), JEOL ECS 400 (100 MHz) or
JEOL ECX 400P (100 MHz) spectrophotometers. Abbreviations of multiplicity are as follows; s: singlet,
d; doublet, t: triplet, q: quartet, sept: septet, m: multiplet, and br: broad. Data are presented as follows;
chemical shift (multiplicity, integration, coupling constant). Assignments were based on H–1H COSY,
1
HMBC and HMQC NMR spectra. Optical rotations were determined on a JASCO P-1010-GT. IR
spectra were acquired on a JASCO FT/IR-460 spectrometer. Mass spectra were recorded on a Thermo
Scientific Exactive. The mass analyzer used for the HRMS measurements was TOF. Melting points
were measured with a Yanaco MP–S3 melting point apparatus. Synthesis and characterization data of 1,
4, 5, 7, 8, ent-27, 28, 32, 34, 35, 51, 67, 68, 71-74, trans-75, cis-75, 76, 79, 80 and 83-88 described in
reference 11 and 13.
Cyclohexyl (2S, 3R)-3-(formylamino)-4-(2,2,2-trichloroethoxycarbonyloxy)-2-
(triisopropylsiloxy)butanoate
A mixture of 8 (103 mg, 0.200 mmol) in CH2Cl2 (1 mL) was treated with TFA (1 mL) at 0 °C for 30
min. The reaction was quenched with sat. aq. NaHCO3, and the mixture was partitioned between AcOEt
ACS Paragon Plus Environment
23

The Journal of Organic Chemistry
Page 24 of 46
and sat. aq. NaHCO3. The organic phase was washed with H2O (×2) and brine, dried (Na2SO4), filtered,
and concentrated in vacuo to afford a crude aminoalcohol. A mixture of the crude aminoalcohol in THF
(2 mL) was treated with N-formylsaccharin (66 mg, 0.30 mmol) for 30 min. N-Formylsaccharin (44 mg,
0.20 mmol) was added to the mixture, and the whole mixture was stirred for 15 min. The reaction was
quenched with sat. aq. NaHCO3, and the mixture was partitioned between AcOEt and sat. aq. NaHCO3.
The organic phase was washed with H2O and brine, dried (Na2SO4), filtered, and concentrated in vacuo
to afford cyclohexyl (2S, 3R)-3-(formylamino)-4-hydroxy-2-(triisopropylsiloxy)butanoate. A mixture of
cyclohexyl (2S, 3R)-3-(formylamino)-4-hydroxy-2-(triisopropylsiloxy)butanoate and NMM (66 ꢀL,
0.60 mmol) in THF (2 mL) was treated with TrocCl (41 ꢀL, 0.30 mmol) at 0 °C for 25 min. The
mixture was warmed to room temperature, and stirred for 45 min. N-Methylmorpholine (NMM, 66 ꢀL,
0.60 mmol) and TrocCl (41 ꢀL, 0.30 mmol) was added to the mixture, which was stirred for 35 min.
The reaction was quenched with sat. aq. NaHCO3, and the mixture was partitioned between AcOEt and
sat. aq. NaHCO3. The organic phase was washed with H2O (×2) and brine, dried (Na2SO4), filtered, and
concentrated in vacuo. The residue was purified by silica gel column chromatography (φ 1×4 cm, 40%
AcOEt/hexane ) to afford cyclohexyl (2S, 3R)-3-(formylamino)-4-(2,2,2-trichloroethoxycarbonyloxy)-
2-(triisopropylsiloxy)butanoate (109 mg, 0.19 mmol, 94% over 3 steps) as a pale yellow oil.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
1H NMR (CDCl3, 500 MHz) δ 8.20 (s, 1H, CHO), 6.08 (d, 1H, NHCHO, JNHCHO, 3 = 8.6 Hz), 4.85-4.67
(m, 5H, CH2CCl3, H-1', H-3), 4.60 (d, 1H, H-2, J2, 3 = 2.3 Hz), 4.38 (dd, 1H, H-4, J4, 3 = 5.7, J4, 4 = 10.6
Hz), 4.27 (dd, 1H, H-4, J4, 3 = 7.5, J4, 4 = 10.6 Hz), 1.91-1.82 (m, 2H, H-2'), 1.78-1.69 (m, 2H, H-2'),
i
1.45-1.11 (m, 6H, H-3', H-4'), 1.10-1.02 (m, 21H, Pr3Si); 13C NMR (CDCl3, 125 MHz) δ 170.7, 170.0,
163.5, 153.6, 94.3, 77.1, 75.1, 74.7, 72.1, 70.5, 67.5, 65.8, 54.3, 49.7, 31.8, 31.7, 31.5, 25.3, 24.0, 23.9,
18.1, 18.0, 12.6, 12.5; ESIMS-LR m/z: [M+Na]+ 600.15; HRMS (ESI-TOF) m/z: [M+Na]+ calcd for
C23H40O7NCl3NaSi 598.1539, found 598.1532; [α]20D +5.18 (c 0.54, CHCl3).
Formyl-(3S)-3-(triisopropylsiloxy)-Asp(O-cyclohexyl)-Oallyl
A solution of crude cyclohexyl (2S, 3R)-3-(formylamino)-4-hydroxy-2-(triisopropylsiloxy)butanoate
(45.2 mg, 0.11 mmol) in acetone (2 mL) was treated with 2.5 M Jones reagent (132 ꢀL, 0.33 mmol) at 0
i
°C for 4 h. The reaction was quenched with PrOH (1.0 mL), and the mixture was partitioned between
Et2O and 1 M aq. HCl. The organic phase was washed with brine, dried (Na2SO4), filtered, and
concentrated in vacuo to afford a crude carboxylic acid. A mixture of the crude carboxylic acid and
Cs2CO3 (53.8 mg, 0.165 mmol) in DMF (1 mL) was treated with allyl bromide (14.3 ꢀL, 0.165 mmol)
at room temperature for 14 h. The mixture was partitioned between AcOEt and H2O. The organic phase
was washed with brine, dried (Na2SO4), filtered, and concentrated in vacuo. The residue was purified by
ACS Paragon Plus Environment
24

Page 25 of 46
The Journal of Organic Chemistry
high-flash silica gel column chromatography (3-10-20% AcOEt/hexane) to afford the title compound
(12.1 mg, 0.027 mmol, 24% over 4 steps) as a colorless oil.
1
2
3
1H NMR (CDCl3, 500 MHz) δ 8.25 (s, 1H, CHO), 6.29 (d, 1H, β-hydroxy-Asp-NH, Jβ-hydroxy-Asp-NH, β-
hydroxy-Asp-α-CH = 9.7 Hz), 5.92 (dddd, 1H, Hc, JHc, Ha = 17.2, JHc, Hb = 10.3, JHc, H-1 = JHc, 1 = 6.3 Hz), 5.35
(dd, 1H, Ha, JHa, Hc = 17.2, JHa, H = 1.8 Hz), 5.28 (dd, 1H, Hb, JHb, Hc = 10.3, JHb, H-1 = 1.8 Hz), 5.16 (dd,
1H, β-hydroxy-Asp-α-CH, Jβ-hydroxy-Asp-α-CH, β-hydroxy-Asp-β-CH = 1.7, Jβ-hydroxy-Asp-α-CH, hydroxy-Asp-NH = 9.7
Hz), 4.98 (d, 1H, β-hydroxy-Asp-β-CH, Jβ-hydroxy-Asp-β-CH, β-hydroxy-Asp-α-CH = 1.7 Hz), 4.81-4.74 (m, 1H, H-
1'), 4.69 (dd, 1H, H-1, J1, Hc = 6.3, J1, 1 =13.2 Hz), 4.59 (dd, 1H, H-1, J1, Hc = 6.3, J1, 1 =13.2 Hz), 1.90-
1.81 (m, 2H, H-2'), 1.76-1.69 (m, 2H, H-2'), 1.58-1.51 (m, 1H, H-4'), 1.45-1.18 (m, 5H, H-3', H-4'),
1.16-1.01 (m, 21H, iPr3Si); 13C NMR (CDCl3, 125 MHz) δ 169.9, 168.7, 160.8, 131.3, 119.7, 74.9, 72.8,
66.9, 54.4, 31.7, 31.5, 25.3, 24.0, 24.0, 18.0, 18.0, 12.6; ESIMS-LR: m/z [M+Na]+ 478.26; HRMS
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(ESI-TOF) m/z: [M+Na]+ calcd for C23H41O6NNaSi 478.2595, found 478.2601; [α]22 +7.94 (c 0.40,
D
CHCl3).
Allyl cyclohexyl (2S, 3S)-2-isocyano-3-(triisopropylsiloxy)butandioate (9)
A mixture of formyl-(3S)-3-(triisopropylsiloxy)-Asp(O-cyclohexyl)-Oallyl (4.3 mg, 0.016 mmol) and
NMM (15.8 ꢀL, 0.14 mmol) in CH2Cl2 (300 ꢀL) was treated with triphosgene (5.7 mg, 0.019 mol) at
−78 °C for 1.5 h. The reaction was quenched with sat. aq. NaHCO3 (1 mL), and the mixture was
partitioned between AcOEt and sat. aq. NaHCO3. The organic phase was washed with H2O (×2) and
brine, dried (Na2SO4), filtered, and concentrated in vacuo to afford a crude isocyanide 9 as a pale yellow
oil. This compound was directly used to the next reaction without further purification.
Cyclohexyl (2S, 3R)-4-(2,2,2-trichloroethoxycarbonyloxy)-3-isocyano-2-(triisopropylsiloxy)butanoate
(10)
A mixture of cyclohexyl (2S, 3R)-3-(formylamino)-4-(2,2,2-trichloroethoxycarbonyloxy)-2-
(triisopropyl-siloxy)butanoate (11.5 mg, 0.020 mmol) and NMM (13.4 ꢀL, 0.12 mmol) in CH2Cl2 (400
ꢀL) was treated with triphosgene (4.7 mg, 0.016 mmol) at −78 °C for 45 min. The reaction was
quenched with sat. aq. NaHCO3 (1 mL), and the mixture was partitioned between AcOEt and sat. aq.
NaHCO3. The organic phase was washed with H2O (×2) and brine, dried (Na2SO4), filtered, and
concentrated in vacuo. The residue was purified by silica gel column chromatography (φ 0.5 × 2 cm,
10% AcOEt/hexane) to afford compound 10 (9.8 mg, 0.018 mmol, 90%) as a colorless oil.
1H NMR (CDCl3, 500 MHz) δ 4.96 (dddd, 1H, H-1', J1', 2' = J1', 2' = 9.2, J1', 2' = J1', 2' = 4.0 Hz), 4.80 (s,
2H, CH2CCl3), 4.57 (d, 1H, H-2, J2, 3 = 3.4 Hz), 4.51 (dd, 1H, H-4, J4, 3 = 6.9, J4, 4 = 10.9 Hz), 4.46 (dd,
1H, H-4, J4, 3 = 5.7, J4, 4 = 10.9 Hz), 4.23 (ddd, 1H, H-3, J3, 2 =3.4 Hz, J3, 4 = 5.7, J3, 4 = 6.9 Hz), 1.97-
i
1.87 (m, 2H, H-2'), 1.78-1.72 (m, 2H, H-2'), 1.51-1.12 (m, 6H, H-3', H-4'), 1.11-1.03 (m, 21H, Pr3Si);
ACS Paragon Plus Environment
25

The Journal of Organic Chemistry
Page 26 of 46
13C NMR (CDCl3, 100 MHz) δ 168.9, 161.6, 153.4, 94.1, 75.2, 71.2, 70.0, 56.3, 31.7, 31.6, 25.3, 23.9,
18.0, 12.6; IR (neat) ν 2138.67 cm-1; ESIMS-LR m/z: [M+Na]+ 580.14; HRMS (ESI-TOF) m/z:
[M+Na]+ calcd for C23H38O6NCl3NaSi 580.1426, found 580.1426; [α]22D +1.98 (c 1.00, CHCl3).
1
2
3
4
5
6
Boc-D-Ser(O-triisopropylsilyl)-OH (11)
7
8
9
A suspension of 24 (1.05 g, 10.0 mmol) and sat. aq. NaHCO3 (40 mL) in THF (80 mL) was treated
with (Boc)2O (3.22 mL, 14.0 mmol) at 0 °C for 10 min. The mixture was warmed to room temperature,
and stirred for 13 h. Di-tert-butyl dicarbonate (0.92 mL, 4.0 mmol) was added to the mixture, which was
stirred for 2 h. The mixture was partitioned between hexane and H2O, and the aqueous phase was
saturated with Na2SO4. The aqueous phase was acidified with 1 M aq. HCl, and extracted with AcOEt.
The organic phase was dried (Na2SO4), filtered, and concentrated in vacuo to afford a crude alcohol. A
mixture of the crude alcohol and imidazole (4.77 g, 70 mmol) in CH2Cl2 (100 mL) was treated with
TIPSCl (7.46 mL, 35 mmol) at 0 °C for 10 min. The mixture was warmed to room temperature, and
stirred for 4 h. Imidazole (0.68 g, 10 mmol) and TIPSCl (1.07 mL, 5 mmol) was added to the mixture,
which was stirred for 2 h. The mixture was concentrated in vacuo, and the residue was partitioned
between AcOEt and 1 M aq. HCl. The organic phase was washed with sat. aq. NaHCO3 and brine, dried
(Na2SO4), filtered, and concentrated in vacuo to afford a crude silyl ester. A mixture of the crude silyl
ester in THF/MeOH (60 mL/30 mL) was treated with K2CO3 (1.93 g, 14 mmol) at room temperature for
30 min. Water (20 mL) was added to the mixture, which was stirred for 1.5 h. The mixture was
partitioned between hexane and H2O. The aqueous phase was extracted with CHCl3/MeOH = 9/1, and
the organic phase was dried (Na2SO4), filtered, and concentrated in vacuo. The residue was diluted with
AcOEt, which was washed with 0.5 M aq. HCl/brine = 1/1 and 1 M aq. HCl, dried(Na2SO4), filtered,
and concentrated in vacuo to afford 11 (3.46 g, 9.96 mmol, quant. over 3 steps) as a white solid.
mp: 64-65 °C; 1H NMR (CDCl3, 500 MHz) δ 5.36 (d, 1H, D-Ser-NH, JD-Ser-NH, D-Ser-α-CH = 9.6 Hz), 4.39
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(dd,1H,
CH, J D-Ser-β-CH
= 9.6, J D-Ser-α-CH
D
-Ser-β-CH, J D-Ser-β-CH
,
D-Ser-β-CH = 10.3, J D-Ser-β-CH
,
D-Ser-α-CH = 9.6 Hz), 4.19 (dd, 1H,
D
-Ser-β-
,
D-Ser-β-CH = 10.3, J D-Ser-β-CH
,
D-Ser-α-CH = 4.0 Hz), 3.91 (dd,1H,
D
-Ser-α-CH, J D-Ser-α-CH
,
D-
t
i
13
,
= 4.0 Hz), 1.45 (s, 9H, Bu), 1.18-0.89 (m, 21H, Pr3Si); C NMR
Ser-β-CH
D-Ser-β-CH
(CDCl3, 125 MHz) δ 176.0, 155.7, 80.3, 64.0, 55.5, 28.4, 17.9, 11.9; ESIMS-LR m/z: [M-H]− 360.41;
HRMS (ESI-TOF) m/z: [M+Na]+ calcd for C17H35O5NNaSi 384.2182, found 384.2199; [α]20D −33.51 (c
0.88, CHCl3).
Cyclohexyl (2S,3R)-3-{[Boc-
[(2,2,2-trichloroethoxycarboxyl)oxy]-2-triisopropylsiloxybutanoate (trans-13)
Cyclohexyl (2S,3R)-3-{[Boc- -Ser(O-triisopropylsilyl)-(3S)-3-(triisopropylsiloxy)-D-Pro]amino}-4-
D-Ser(O-triisopropylsilyl)-(3S)-3-(triisopropylsiloxy)-Pro]amino}-4-
D
[(2,2,2-trichloroethoxycarboxyl)oxy]-2-triisopropylsiloxybutanoate (cis-13)
ACS Paragon Plus Environment
26

Page 27 of 46
1
2
3
4
5
6
7
The Journal of Organic Chemistry
A solution of 4 (52 mg, 0.24 mmol) and 11 (289 mg, 0.80 mmol) in HFIP (1 mL) was treated with a
solution of 10 (88 mg, 0.24 mmol) in HFIP (2 mL) at room temperature for 24 h. The mixture was
concentrated in vacuo. The residue was purified by high-flash silica gel column chromatography (0-20%
acetone/hexane) to afford trans-13 (105 mg, 0.0904 mmol, 57%) as a colorless amorphous and cis-13
(32 mg, 0.028 mmol, 17%) as a colorless amorphous.
8
1
9
Data for trans-13: H NMR (CDCl3, 400 MHz) δ 7.17 (d, 1H, C-3-NH, JC-3-NH, H-3 = 9.2 Hz), 5.58 (d,
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
1H,
1H, 3-hydroxy-Pro-β-CH, J3-hydroxy-Pro-β-CH, 3-hydroxy-Pro-γ-CH = 3.4 Hz), 4.62-4.53 (m, 4H,
H-2), 4.35 (s, 1H, 3-hydroxy-Pro-α-CH), 4.25 (d, 2H, H-4, JH-4, H-3 = 6.9 Hz), 3.95 (dd, 1H,
D
-Ser-NH, JD-Ser-NH, D-Ser-α-CH = 8.1 Hz), 4.78-4.69 (m, 1H, H-1'), 4.77 (s, 2H, CH2CCl3), 4.67 (br d,
-Ser-β-CH, H-3,
-Ser-α-
D
D
CH, JD-Ser-α-CH, D-Ser-β-CH = 4.6, JD-Ser-α-CH, D-Ser-β-CH = 9.8 Hz), 3.82-3.75 (m, 2H, 3-hydroxy-Pro-δ-CH),
2.21-2.13 (m, 1H, 3-hydroxy-Pro-γ-CH), 1.94 (dd, 1H, 3-hydroxy-Pro-γ-CH, J3-hydroxy-Pro-γ-CH, 3-hydroxy-Pro-
t
= 5.7, J3-hydroxy-Pro-γ-CH, 3-hydroxy-Pro-γ-CH = 13.2 Hz), 1.86-1.48 (m, 4H, H-2'), 1.43 (s, 9H, Bu), 1.38-
δ-CH
i
13
1.20 (m, 6H, H-3', H-4'), 1.19-0.95 (m, 63H, Pr3Si); C NMR (CDCl3, 125 MHz) δ 170.8, 170.4,
169.5, 155.4, 153.5, 94.5, 79.4, 74.1, 73.6, 70.6, 69.6, 65.9, 63.7, 54.3, 50.7, 45.6, 34.1, 31.6, 31.3, 28.5,
25.3, 23.9, 23.8, 18.1, 12.5, 12.1, 12.0; ESIMS-LR m/z: [M+Na]+ 1184.55; HRMS (ESI-TOF) m/z:
[M+Na]+ calcd for C53H100O12N3Cl3NaSi3 1182.5573, found 1182.5568; [α]20D −6.85 (c 0.59, CHCl3).
1
Data for cis-13: H NMR (CDCl3, 400 MHz, a mixture of several rotamers at 20 °C, selected data for
the major rotamer) δ 6.60 (d, 1H, C-3-NH, JC-3-NH, H-3 = 7.6 Hz), 5.09 (d, 1H,
D
-Ser-NH, JD-Ser-NH, D-Ser-α-
= 8.6 Hz), 4.85-4.69 (m, 4H, CH2CCl3, H-2, H-1'), 4.62-4.52 (m, 2H, 3-hydroxy-Pro-β-CH, H-3),
CH
4.49-4.28 (m, 2H,
6.9 Hz), 4.25 (d, 1H, H-4, JH-4, H-3 = 6.9 Hz), 4.26 (dd, 1H, H-4, JH-4, H-3 = 5.8, JD-Ser-β-CH, D-Ser-β-CH = 10.3
Hz), 3.99 (dd, 1H, -Ser-α-CH, JD-Ser-α-CH, D-Ser-β-CH = 5.3, JD-Ser-α-CH, D-Ser-β-CH = 10.3 Hz), 3.90-3.81 (m,
D
-Ser-β-CH ), 4.37 (d, 1H, 3-hydroxy-Pro-α-CH, J3-hydroxy-Pro-α-CH, 3-hydroxy-Pro-β-CH =
D
1H, 3-hydroxy-Pro-δ-CH), 3.81-3.68 (m, 1H, H-4, 3-hydroxy-Pro-δ-CH), 2.37-2.27 (m, 1H, 3-hydroxy-
Pro-γ-CH), 2.19-2.11 (m, 1H, 3-hydroxy-Pro-γ-CH), 1.96-1.87 (m, 2H, H-2'), 1.78-1.67 (m, 2H, H-2'),
t
i
13
1.41 (s, 9H, Bu), 1.49-1.20 (m, 6H, H-3', H-4'), 1.18-0.97 (m, 63H, Pr3Si); C NMR (CDCl3, 125
MHz) δ 171.2, 171.1, 170.9, 168.6, 168.1, 167.9, 155.5, 154.6, 153.6, 153.4, 94.5, 89.3, 79.6, 79.2, 79.0,
77.1, 75.2, 74.8, 74.6, 73.7, 73.2, 71.6, 70.3, 70.2, 65.9, 65.3, 64.2, 63.8, 63.1, 60.1, 53.9, 53.5, 51.7,
51.1, 45.0, 44.7, 43.8, 33.3, 32.7, 31.8, 31.8, 31.6, 30.5, 28.5, 28.4, 25.4, 24.2, 24.2, 24.1, 18.2, 18.1,
18.0, 12.5, 12.4, 12.3, 12.2, 12.0, 12.0; ESIMS-LR m/z: [M+Na]+ 1184.55; HRMS (ESI-TOF) m/z:
[M+Na]+ calcd for C53H100O12N3Cl3NaSi3 1182.5573, found 1182.5570; [α]20D +8.48 (c 0.59, CHCl3).
(S)-5-[(Benzyloxycarbonyl)amino]-2-(formylamino)pentyl 2,2,2-trichloroethyl carbonate (26)
Alcohol 25 (2.11 g, 6.00 mmol) was treated with 4 M HCl/dioxane (30 mL) at room temperature for
15 min. The mixture was concentrated in vacuo to afford a crude amine hydrochloride salt. A solution
ACS Paragon Plus Environment
27

The Journal of Organic Chemistry
Page 28 of 46
of the crude amine hydrochloride salt and Et3N (904 ꢀL, 6.00 mmol) in THF (30 mL) was treated with
N-formylsaccharin (1.39 g, 6.60 mmol) at room temperature for 40 min. The reaction was quenched
with sat. aq. NaHCO3, and the mixture was partitioned between AcOEt and 1 M aq. HCl. The organic
phase was washed with H2O and brine, dried (Na2SO4), filtered, and concentrated in vacuo to afford a
crude formamide. A mixture of the crude formamide and NMM (1.45 mL) in THF (30 mL) was treated
with TrocCl (909 ꢀL, 6.60 mmol) at 0 °C for 10 min. The mixture was partitioned between AcOEt and
H2O. The organic phase was washed with 1 M aq. HCl, H2O and brine, dried (Na2SO4), filtered, and
concentrated in vacuo. The residue was purified by high-flash silica gel column chromatography (0-1-
2% MeOH/CHCl3) to afford 26 (1.67 g, 3.66 mmol, 61% over 3 steps) as a yellow oil.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
1H NMR (CDCl3, 400 MHz) δ 8.19 (s, 1H, CHO), 7.41-7.29 (m, 5H, Ph), 5.97 (d, 1H, NHCHO, JNHCHO,
= 7.7 Hz), 5.09 (s, 2H, PhCH2), 4.98-4.89 (m, 1H, C-5-NH), 4.77 (s, 2H, CH2CCl3), 4.38-4.19 (m,
H-2
13
3H, H-1, H-2), 3.28-3.13 (m, 2H, H-5), 1.68-1.48 (m, 4H, H-3, H-4); C NMR (CDCl3, 125 MHz) δ
164.7, 161.5, 156.7, 153.8, 153.7, 136.5, 128.5, 128.1, 127.9, 94.3, 94.2, 76.8, 70.6, 70.0, 66.5, 51.4,
46.7, 40.4, 40.2, 28.2, 27.9, 26.3, 26.1; ESIMS-LR m/z: [M+H]+ 455.95; HRMS (ESI-TOF) m/z:
[M+H]+ calcd for C17H22O6N2Cl3 455.0543, found 455.0542; [α]20D −11.19 (c 0.95, CHCl3).
(S)-5-[(Benzyloxycarbonyl)amino]-2-isocyanopentyl 2,2,2-trichloroethyl carbonate (19)
A solution of compound 26 (547 mg, 1.20 mmol) and NMM (793 ꢀL, 7.20 mmol) in CH2Cl2 (8 mL)
was treated with triphosgene (285 mg, 0.96 mmol) at −78 °C for 30 min. The mixture was partitioned
between AcOEt and sat. aq. NaHCO3. The organic phase was washed with sat. aq. NaHCO3, H2O and
brine, dried (Na2SO4), filtered, and concentrated in vacuo. The residue was purified by silica gel column
chromatography (φ 2 cm × 6 cm, 35% AcOEt/hexane) to afford 19 (442 mg, 1.01 mmol, 84%) as a pale
yellow oil.
1H NMR (CDCl3, 400 MHz) δ 7.49-7.22 (m, 5H, Ph), 5.10 (s, 2H, PhCH2), 4.86-4.72 (m, 3H, CH2CCl3,
C-5-NH), 4.28 (d, 2H, H-1, J1, 2 = 5.5 Hz), 3.99-3.88 (m, 1H, H-2), 3.33-3.19 (m, 2H, H-5), 1.82-1.64
(m, 4H, H-3, H-4); 13C NMR (CDCl3, 100 MHz) δ 158.8, 156.6, 153.6, 136.5, 128.7, 128.3, 128.2, 94.1,
68.9, 66.9, 53.1, 40.0, 28.3, 26.3; IR (neat) ν 2140.60 cm-1; ESIMS-LR m/z: [M+Na]+ 469.07; HRMS
(ESI-TOF) m/z: [M+H]+ calcd for C17H20O5N2Cl3 437.0432, found 437.0429; [α]22 −0.88 (c 1.00,
D
CHCl3).
Boc-allo-D-Thr(O-triisopropylsilyl)-OBn (30)
A suspension of 29 (595 mg, 5.0 mmol) and sat. aq. NaHCO3 (7.5 mL) in THF (15 mL) was treated
with (Boc)2O (2.07 mL, 9.0 mmol) at room temperature for 4 d. The mixture was partitioned between
hexane and H2O, and the aqueous phase was saturated with Na2SO4. The aqueous phase was acidified
with 1 M aq. HCl, and extracted with AcOEt. The organic phase was dried (Na2SO4), filtered, and
ACS Paragon Plus Environment
28

Page 29 of 46
The Journal of Organic Chemistry
concentrated in vacuo to afford a crude carboxylic acid. A solution of the crude carboxylic acid and
K2CO3 (828 mg, 6.0 mmol) in DMF (25 mL) was treated with BnBr (713 ꢀL, 6.0 mmol) at room
temperature for 24 h. The mixture was partitioned between AcOEt and H2O. The organic phase was
washed with brine, dried (Na2SO4), filtered, and concentrated in vacuo to afford a crude alcohol. A
solution of the crude alcohol and 2,6-lutidine (1.16 mL, 10.0 mmol) in CH2Cl2 (25 mL) was treated with
TIPSOTf (1.48 mL, 5.5 mmol) at −78 °C. Then the mixture was warmed to −50 °C, and stirred for 190
min. The reaction was quenched with MeOH (2 mL), and partitioned between AcOEt and sat. aq.
NaHCO3. The organic phase was washed with 1 M aq. HCl, sat. aq. NaHCO3, H2O and brine, dried
(Na2SO4), filtered, and concentrated in vacuo. The residue was purified by high-flash silica gel column
chromatography (3-20% AcOEt/hexane) to afford 30 (1.93 g, 4.14 mmol, 83% over 3 steps) as a
colorless oil.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
1H NMR (CDCl3, 500 MHz) δ 7.38-7.29 (m, 5H, Ph), 5.37 (d, 1H, allo-
-Thr-NH, Jallo-D-Thr-NH, allo-D-Thr-
D
= 8.0 Hz), 5.23 (d, 1H, PhCH, JPhCH, PhCH = 12.1 Hz), 5.15 (d, 1H, PhCH, JPhCH, PhCH = 12.1 Hz),
α-CH
t
4.36-4.28 (m, 2H , allo-D-Thr-α-CH, allo-
D
-Thr-β-CH), 1.43 (s, 9H, Bu), 1.25 (d, 3H, allo- D-Thr-γ-
i
13
CH, Jallo-D-Thr-γ-CH, allo-D-Thr-γ-CH = 6.3 Hz), 1.08-1.01 (m, 21H, Pr3Si); C NMR (CDCl3, 125 MHz) δ
170.2, 155.4, 135.6, 128.6, 128.4, 79.8, 70.3, 67.0, 60.1, 28.4, 20.7, 18.1, 18.1, 12.6; ESIMS-LR m/z:
[M+H]+ 466.40; HRMS (ESI-TOF) m/z: [M+H]+ calcd for C25H44O5NSi 466.2989, found 466.3006;
[α]20D −20.10 (c 0.75, CHCl3).
(R)-(3-Hydroxy-14-methylpentadecanyl)-allo-D-Thr(O-triisopropylsilyl)-OBn (36)
Compound 30 (373 mg, 0.80 mmol) was treated with 25% TFA/CH2Cl2 (8 mL) at room temperature
for 10 min. The reaction was quenched with sat. aq. NaHCO3, and the mixture was partitioned between
AcOEt and H2O. The organic phase was washed with brine, dried (Na2SO4), filtered, and concentrated
in vacuo to afford crude amine 23. A solution of the crude amine 23, 35 (218 mg, 0.80 mmol), NaHCO3
(202 mg, 2.40 mmol) and HOAt (327 mg, 2.40 mmol) in CH2Cl2 (8.0 mL) was treated with EDCI (460
mg, 2.40 mmol) at room temperature for 60 min. The mixture was partitioned between AcOEt and sat.
aq. NaHCO3, and the organic phase was washed with 1 M aq. HCl, H2O and brine, dried (Na2SO4),
filtered, and concentrated in vacuo. The residue was purified by high-flash silica gel column
chromatography (30% AcOEt/hexane) to afford 36 (425 mg, 0.69 mmol, 86%) as a colorless oil.
1H NMR (CDCl3, 500 MHz) δ 7.37-7.31 (m, 5H, Ph), 6.57 (d, 1H, allo-
= 7.7 Hz), 5.24 (d, 1H, PhCH, JPhCH, PhCH = 12.0 Hz), 5.15 (d, 1H, PhCH, JPhCH, PhCH = 12.0 Hz),
D
-Thr-NH, Jallo-D-Thr-NH, allo-D-Thr-
α-CH
4.62 (dd, 1H, allo-D-Thr-α-CH, Jallo-D-Thr-α-CH, allo-D-Thr-NH = 7.8, Jallo-D-Thr-α-CH, allo-D-Thr-β-CH = 2.8 Hz),
4.31 (dq, 1H, allo-D-Thr-β-CH, Jallo-D-Thr-β-CH, allo-D-Thr-α-CH = 2.8, Jallo-D-Thr-β-CH, allo-D-Thr-γ-CH = 6.4 Hz),
3.98-3.88 (m, 1H, H-3), 3.50 (d, 1H, OH, JOH, 3 = 3.2 Hz), 2.37 (dd, 1H, H-2, J2, 2 = 15.1, J2, 3 = 2.8 Hz),
ACS Paragon Plus Environment
29